Microlithography projection optical system, tool and method of production

ABSTRACT

A microlithography projection optical system is disclosed. The system can include a plurality of optical elements arranged to image radiation having a wavelength λ from an object field in an object plane to an image field in an image plane. The plurality of optical elements can have an entrance pupil located more than 2.8 m from the object plane. A path of radiation through the optical system can be characterized by chief rays having an angle of 3° or more with respect to the normal to the object plane. This can allow the use of face shifting masks as objects to be imaged, in particular for EUV wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP2007/000068, filed Jan. 5, 2007, which is a continuation-in-partof International Application No. PCT/EP2006/008869, filed Sep. 12, 2006.International Application No. PCT/EP2007/000068 also claims the benefitof U.S. Ser. No. 60/793,387, filed Apr. 7, 2006. InternationalApplication No. PCT/EP2007/000068 is incorporated by reference herein inits entirety.

FIELD

This disclosure relates to a microlithography projection optical system,such as a projection objective, a microlithographic tool including suchan optical system, a method for microlithographic production ofmicrostructured components using such a microlithographic tool and amicrostructured component produced by such a method.

BACKGROUND

Projection objectives are widely used in microlithography to transfer apattern from a reticle to a substrate by forming an image of the reticleon a layer of a photosensitive material disposed on the substrate. Ingeneral, projection objectives fall into three different classes:dioptric objectives; catoptric objectives; and catadioptric objectives.Dioptric objectives use refractive elements (e.g., lens elements) toimage light from an object plane to an image plane. Catoptric objectivesuse reflective elements (e.g., mirror elements) to image light from anobject plane to an image plane. Catadioptric objectives use bothrefractive and reflective elements to image light from an object planeto an image plane.

SUMMARY

In some embodiments, the disclosure provides a microlithographyprojection optical system that is flexible with respect to differenttypes of masks or reticles to be imaged.

In one aspect, the disclosure features a microlithography projectionoptical system that includes a plurality of optical elements arranged toimage radiation from an object field in an object plane to an imagefield in an image plane. The system has an entrance pupil located morethan 2.8 m from the object plane. A path of the radiation of the systemhas chief rays that are at an angle of 3° or more with respect to anormal at the object plane.

In another aspect, the disclosure features a microlithography tool thatincludes an illumination system and a microlithography projectionoptical system as described in the pre-ceding paragraph.

In a further aspect, the disclosure features a EUV microlithography toolthat includes an optical projection system including a plurality ofoptical elements arranged to image radiation having a wave length λ froman object field in an object plane of the optical projection system toan image field in an image plane of the optical projection system. Theoptical projection system is telecentric at the object plane of theoptical projection system. The microlithography tool also includes aradiation source configured to provide the radiation at λ to the objectplane of the optical projection system, where λ is 30 nm or less. Inaddition, the microlithography tool includes an illumination systemhaving one or more elements arranged to direct radiation from theradiation source to the object plane of the optical projection system.

In an additional aspect, the disclosure features a method that includesusing a tool as described in either of the two preceding paragraphs toproduce micro-structured components.

In some embodiments, due in part at least to the entrance pupil of theoptical system being located more than 2.8 m from the object plane, thechief rays are substantially parallel to each other at the object plane.Uniformity of the chief ray directions at the reticle can reduce oravoid field dependency of shading effects at the reticle. These shadingeffects can lead to a unwanted field dependency of the imagingproperties of projection objective, such as, for example, the resolutionlimit. Accordingly, uniformity of the chief ray directions can reducethese field dependent imaging properties, providing an image that hasimproved uniformity across the field. Furthermore, chief rays beingsubstantially parallel to each other and to the normal of the objectplane allow the use of phase shifting masks, especially to be imaged byEUV wavelengths. An angle between the normal at the object plane and thechief rays of 3° or more makes possible both a separation of anillumination ray path and a projection ray path. This can facilitate theuse of a reflective object to be imaged.

In general, the image plane of the optical system is parallel to theobject plane. The projection objective may include four or more (e.g.,six or more) reflective elements. The projection objective may includethree, four, five or six elements that are reflective elements havingrotationally asymmetric surfaces positioned in a path of the radiation.The optical system may have a field at the image plane having a minimumradius of curvature of 300 mm. For a meridional section of the opticalsystem, the chief rays may have a maximum angle of incidence on asurface of each of the elements of less than 20° (e.g., less than 18°,less than 15°). Due to its telecentricity at the object side, theoptical system has an entrance pupil located at infinity. Imagedradiation may be reflected from an object positioned at the objectplane. Such an object may be a phase shift mask. In general the objectpositioned at the object plane is a reticle that is imaged by theplurality of elements to the image plane. The optical system may have ademagnification of 4×. The plurality of elements will be arranged toimage the radiation to an intermediate image plane between the objectplane and the image plane. In this case, a field stop may be providedpositioned at or near the intermediate image plane. The projectionobjective may include five elements and an intermediate image plane maybe located between a fourth element and a fifth element along the pathof the radiation from the object plane to the image plane. The objectand image planes may be separated by a distance L being about 1 m ormore. The optical path length of the radiation from the object plane tothe image plane may be about 2 L or more (e.g., about 3 L or more, about4 L or more). The projection objective may include at least one pair ofadjacent elements in the path of the radiation, where the pair ofadjacent elements is separated by about 0.5 L or more. Advantageously,none of the plurality of elements causes an obscuration of the exitpupil at the image plane. The plurality of elements may include four ormore elements that have free boards of about 25 mm or less. The four ormore elements may have free boards of about mm or more. The plurality ofelements may include a first mirror and a second mirror, the first andsecond mirrors having a minimum distance from the object plane of d₁ andd₂, respectively, where d₁/d₂ is about 2 or more or where d₁/d₂ is lessthan 2. The plurality of elements may include a first element in thepath of the radiation from the object plane to the image plane, wherethe first element has positive optical power. The optical system mayinclude an aperture stop positioned between the object plane and theimage plane. The plurality of elements of the optical system may includethree elements and the aperture stop may be positioned between thesecond and third elements in the path of the radiation from the objectplane to the image plane. Alternatively, the aperture stop may bepositioned at the second element or at the third element or at someother position in the projection lens, e.g. between the first and thesecond element. The radiation may pass through the aperture stop once ortwice. A radiation source which is used with the optical systemaccording to the disclosure may be a laser radiation source having awavelength of about 300 nm or less (e.g., 200 nm or less, 100 nm orless).

In some embodiments, in a telecentric optical system, the chief rays atthe object plane are parallel to each other within a certain deviationlimit. For example, the chief rays can be parallel to each other towithin about 0.5° or less (e.g., about 0.4° or less, about 0.3° or less,about 0.2° or less, about 0.1° or less, about 0.05° or less, 0.01° orless) at the object plane.

In certain embodiments, the chief angle to the normal at the objectplane can provide a good separation between the illumination andprojection ray path. The higher this angle is, the more separation ispossible at a given distance. Advantageously, the angle between thechief rays and the object plane normal is 5° or more (7° or more).

In some embodiments, the system can include a reflective object, such asa phase shift mask. This can exploit advantageously the possibilities ofthe optical system.

In certain embodiments, the projection objective is a catoptricprojection objective. Such a projection objective having exclusivelyreflective components can be used with ultra short wavelengths, inparticular with EUV wavelengths being below 30 mm.

In the following specification, a rotationally asymmetric surfaceaccording to the disclosure also is referred to as a freeform surface.Unlike spherical or aspherical mirrors, free-form surfaces do not havean axis of rotational symmetry. Freeform surfaces according to thepresent disclosure differ from known aspheric rotational symmetricmirror surfaces for EUV projection objectives in that such knownaspheric mirror surfaces are described via a mathematical Taylorexpansion, i.e. having a sag being given by a rotational symmetricpolynomial of grade n. The center point of this Taylor expansion for allthese polynomial terms is defined by a common optical axis. Known mirrorsurfaces are described by such an expansion, because the Taylorexpansion is easy to calculate, easy to optimize and there exists a lotof experience in manufacturing such mirror surfaces. However, it wasrealized by the inventors that the known Taylor expansion with commoncenter leads to an unwanted distortion which cannot be lowered below acertain level. This distortion limitation being inherent to rotationalsymmetric optical surfaces is avoided, when according to the disclosureone of the optical surfaces is embodied as free-form or rotationallyasymmetric surface. In some embodiments, a freeform surface may be asurface being mirror symmetric to a meridional plane of the opticalsystem. In certain embodiments, a deviation of the rotationallyasymmetric surface from a best-fit rotationally symmetric surface cangive the possibility to eliminate higher aberration values, inparticular aberrations being in the order of several wavelengths of theilluminating radiation.

A mathematical expansion of the freeform surface according to someembodiments can give a good and reproducible manufacturing of thereflecting surfaces. In this expansion, α may be 66, for example.Further, m may consist of even integers, for example. Further, m+n maybe equal or bigger than 10, for example.

A deviation according to certain embodiments can provide for a properreduction of the objective's distortion below the limit which isreachable using rotationally symmetric optical surfaces. The rotationalasymmetric surface may deviate from the best-fit rotational symmetricsurface by about 50 nm or more (e.g., about 100 nm or more, about 500nm, about 1000 nm or more) at the one or more locations.

In some embodiments, the plurality of reflective elements define ameridional plane, and the elements are mirror symmetric with respect tothe meridional plane. In such embodiments, for example, restrictions onproducing a free-form optical surface may be reduced.

In some embodiments, two reflective elements having optical surfaces canlead to the possibility of a better aberration minimization or give thepossibility to meet such an aberration minimization desired propertywith freeform surfaces being less complicated to manufacture. Theoptical system also may have three, four, five or six freeform elements.

In certain embodiments, an optical system having no more than tworeflective elements with a positive chief ray angle magnification canexhibit a relatively low incident ray angles on the mirrors, thuscausing lower aberrations at the outset. In some embodiments, this canhold when using an optical system including only one reflective elementhaving a positive chief ray angle magnification, such as for systemshaving at least one intermediate image.

A numerical aperture of the optical system according to some embodimentscan allow a high resolution. The image-side numerical aperture be ashigh as 0.25 or more (e.g., 0.28 or more, 0.3 or more, 0.35 or more, 0.4or more).

An image field dimension according to some embodiments can enable anefficient use of the optical system in a microlithography projectionapparatus. A rectangular field may have a minimum dimension of about 2mm and may have a first dimension of about 1 mm or more and seconddimension of about 1 mm or more, where the first and second dimensionsare orthogonal and are measured in the image plane. This seconddimension may be about 10 mm or more (e.g., about 20 mm or more).

A distortion according to some embodiments and a wavefront erroraccording to certain embodiments can lead to a projection quality whichonly may be limited by diffraction, i.e. by the wavelength of theprojection light. An optical system with such low distortion inparticular is optimized for use the EUV light sources in the rangebetween 10 and 30 nm.

Within a low limit according to some embodiments parallel chief rayslead to a high quality telecentric optical system at the object plane.

A telecentric optical system according to certain embodiments toleratesheight variations of a substrate due to non flat wafer topographyarranged in the image plane or allows for defocusing of the image planewithout any change in the magnification ratio.

An optical system according to certain embodiments can lead to a veryhigh resolution. The ration θ/NA may be about 60 or less (e.g., 50 orless).

An optical system with an object-image shift of about 75 mm or lessaccording to some embodiments can lead to a slim optical design of theoptical system. The object-image shift may be about 50 mm or less (e.g.,about 25 mm or less). In case of zero object-image shift, the opticalsystem can be rotated about the axis intersecting the center fieldpoints in the object and image fields without the central field pointtranslating. This is in particular advantageous where metrology andtesting tools are used involving rotation of the optical system.

An optical system having a radiation source according to certainembodiments can exploit advantageously the aberration minimization byuse of the at least one freeform surface, as reduction to aberrationsand distortions in the range of the wavelength of such a radiationsource are possible. Optionally, the wavelength is in a range from about10 nm to about 15 nm.

The advantages of an optical system according to some embodiments and ofthe microlithographic tools according to certain embodiments cancorrespond to those mentioned. The same holds with respect to certainembodiments of methods of manufacturing.

Embodiments can include one or more of the following advantages.

In some embodiments, a catoptric projection objectives is telecentric atthe image plane. This can provide for constant or nearly constant imagemagnification over a range of image-side working distances.

In certain embodiments, catoptric projection objectives have extremelyhigh resolution. For example, projection objectives can have thecapability of resolving structures smaller than about 50 nm. Highresolution can be achieved in projection objectives that have a highimage-side numerical aperture that are designed for operation at shortwavelengths (e.g., about 10 nm to about 30 nm).

In some embodiments, a projection objective can provide images with lowaberrations. In certain embodiments, projection objectives are correctedfor wavefront error of about 30 mλ or less. In certain embodiments,projection objectives are corrected for distortion below values of about2 nm or less.

In certain embodiments, a catoptric objective has a high numericalaperture and provides imaging with low image distortion, low wavefronterror, and telecentricity at the image plane over a relatively largeimage field. These features can be achieved by use of one or morefreeform mirrors.

In some embodiments, projection objective metrology can be easilyimplemented despite rotations of the projection objective about arotation axis. For example, embodiments of projection objectives (e.g.,high NA projection objectives) may have relatively small or zeroobject-image shift which result in little or no translation of thecentral field point when the projection objective rotates about theaxis. Thus, when the projection objective is subject to rotation,metrology can be repeatable performed in the same field position withouthaving to relocate that field position.

In certain embodiments, a catoptric projection objection has no fielddependent pupil obscuration or no central pupil obscuration at all.

In some embodiments, a projection objective can be adapted for operationat a variety of different wavelengths, including visible and ultraviolet(UV) wavelengths. Embodiments can be adapted for operation at Extreme UV(EUV) wavelengths. Furthermore, embodiments can be adapted for use atmore than one wavelength, or over a range of wavelengths.

In some embodiments, a catoptric projection objective can be used inlithography tools (e.g., lithography scanners) and can providerelatively low overscan. Low overscan is accomplished, for example, byusing projection objectives with rectangular image fields. In suchembodiments, the image can be aligned so that an edge of the rectangularfield is parallel to the leading edge of the die sites, avoidingscanning the leading edge of the die sites beyond the edge of the imagefield in order to scan the site corners, as is typically the case whenrectangular or square die sites are scanned relative to arcuate fields.

In certain embodiments, lithography tools with relatively highthroughput are provided. For example, embodiments having relatively lowoverscan are more efficient than comparable systems that have largeroverscan. Accordingly, these low overscan systems can provide higherwafer throughput than the comparable systems.

In some embodiments, catoptric projection objectives are provided thathave low or no field dependence of shading effects. For example,catoptric projection objectives can have their entrance pupil locatedfar from the object plane (e.g., at infinity) providing uniformillumination angles of the chief rays on the object field. This canreduce or avoid field dependent shading effects that occurs where chiefray angles vary across the object field. Alternatively, or additionally,projection objectives can have relatively small values chief rayincident angles and/or small variations of incident angles for rays inthe meridional section for each mirror in the projection objective,resulting in an increased average reflectivity of each mirror.

Other features and advantages will be apparent from the description, thedrawings, and the claims.

All or selected features from the claims or subclaims may be combined toform embodiments which are in particular advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a microlithography tool.

FIG. 2 is a schematic view showing a portion of the microlithographytool shown in FIG. 1.

FIG. 3 is a cross-sectional view of a portion of a mirror from aprojection objective shown in meridional section.

FIG. 4 is a schematic view of a ray path at a mirror having a positivechief ray angle magnification.

FIG. 5 is a schematic view of a ray path at a mirror having a negativechief ray angle magnification.

FIG. 6A is a view of a mirror's footprint.

FIG. 6B is a cross-section view of the mirror shown in FIG. 6A.

FIG. 7A is a plan view of an embodiment of a ring segment field.

FIG. 7B is a plan view of a ring segment field relative to a pair ofwafer die sites.

FIG. 7C is a plan view of a rectangular field relative to a pair ofwafer die sites.

FIG. 8 is a schematic view of the projection objective of themicrolithography tool shown in FIG. 1.

FIG. 9 is a cross-sectional view of components of a projection objectiveshown in meridional section.

FIG. 10 is a cross-sectional view of a projection objective shown inmeridional section.

FIG. 11 is a cross-sectional view of a projection objective shown inmeridional section.

FIG. 12 is a cross-sectional view of a projection objective shown inmeridional section.

FIG. 13 is a cross-sectional view of an optical system that includes theprojection objective shown in FIG. 12.

DETAILED DESCRIPTION

In one aspect, the disclosure relates to catoptric projection objectivesthat have one or more mirrors having a freeform mirror surface (referredto as freeform mirrors). Catoptric projection objectives with freeformmirrors can be used in microlithography tools. Referring to FIG. 1, amicrolithography tool 100 generally includes a light source 110, anillumination system 120, a projection objective 101, and a stage 130. ACartesian coordinate system is shown for reference. Light source 110produces radiation at a wavelength λ and directs a beam 112 of theradiation to illumination system 120. Illumination system 120 interactswith (e.g., expands and homogenizes) the radiation and directs a beam122 of the radiation to a reticle 140 positioned at an object plane 103.Projection objective 101 images radiation 142 reflected from reticle 140onto a surface of a substrate 150 positioned at an image plane 102. Theradiation on the image-side of projection objective 101 is depicted asrays 152. As shown in FIG. 1, the rays are illustrative only and notintended to be accurately depict the path of the radiation with respectto reticle 140, for example. Substrate 150 is supported by stage 130,which moves substrate 150 relative to projection objective 101 so thatprojection objective 101 images reticle 140 to different portions ofsubstrate 150.

Projection objective 101 includes a reference axis 105. In embodimentswhere projection objective is symmetric with respect to a meridionalsection, reference axis 105 is perpendicular to object plane 103 andlies inside the meridional section.

Light source 110 is selected to provide radiation at a desiredoperational wavelength, λ, of tool 100. In some embodiments, lightsource 110 is a laser light source, such as a KrF laser (e.g., having awavelength of about 248 nm) or an ArF laser (e.g., having a wavelengthof about 193 nm). Non-laser light sources that can be used includelightemitting diodes (LEDs), such as LEDs that emit radiation in theblue or UV portions of the electromagnetic spectrum, e.g., about 365 nm,about 280 nm or about 227 nm.

Typically, for projection objectives designed for operation inlithography tools, wavelength λ is in the ultraviolet portion, the deepultraviolet portion or the extreme ultraviolet portion of theelectromagnetic spectrum. For example, λ can be about 400 nm or less(e.g., about 300 nm or less, about 200 nm or less, about 100 nm or less,about 50 nm or less, about 30 nm or less). λ can be more than about 2 nm(e.g., about 5 nm or more, about 10 nm or more). In embodiments, λ canbe about 193 nm, about 157 nm, about 13 nm, or about 11 nm. Using arelatively short wavelength may be desirable because, in general, theresolution of a projection objective is approximately proportional tothe wavelength. Therefore shorter wavelengths can allow a projectionobjective to resolve smaller features in an image than equivalentprojection objectives that use longer wavelengths. In certainembodiments, however, λ can be in non-UV portions of the electromagneticspectrum (e.g., the visible portion).

Illumination system 120 includes optical components arranged to form acollimated radiation beam with a homogeneous intensity profile.Illumination system 120 typically also includes beam steering optics todirect beam 122 to reticle 140. In some embodiments, illumination system120 also include components to provide a desired polarization profilefor the radiation beam.

Object plane 103 is separated from image plane 102 by a distance L,which is also referred to as the lengthwise dimension, or tracklength,of projection objective 101. In general, this distance depends on thespecific design of projection objective 101 and the wavelength ofoperation of tool 100. In some embodiments, such as in tools designedfor EUV lithography, L is in a range from about 1 m to about 3 m (e.g.,in a range from about 1.5 m to about 2.5 m). In certain embodiments, Lis less than 2 m, such as about 1.9 m or less (e.g., about 1.8 m orless, about 1.7 m or less, about 1.6 m or less, about 1.5 m or less). Lcan be more than about 0.2 m or more (e.g., about 0.3 m or more, about0.4 m or more, about 0.5 m or more, about 0.6 m or more, about 0.7 m ormore, about 0.8 m or more, about 0.9 m or more, about 1 m or more).

The ratio of the optical path length of imaged radiation to thetracklength varies depending on the specific design of projectionobjective 101. In some embodiments, the ratio of this optical pathlength to tracklength can be relatively high. For example, the ratio ofthis optical path length to tracklength can be about two or more (e.g.,about 2.5 or more, about three or more, about 3.5 or more, about four ormore, about 4.5 or more, about five or more).

Projection objective 101 has a magnification ratio, which refers to theratio of the dimensions of the field at object plane 103 to thecorresponding dimensions of the field at image plane 102. Typically,projection objectives used in lithography tools are reduction projectionobjectives, meaning they reduce the dimensions of, or demagnify, theimage.

In some embodiments, therefore, projection objective 101 can produce afield at image plane 102 whose dimensions are reduced by about 2× ormore (e.g., about 3× or more, about 4× or more, about 5× or more, about6× or more, about 7× or more, about 8× or more, about 9× or more, about10× or more) compared to the dimensions at object plane 103. In otherwords, projection objective 101 can have a demagnification of about 2×or more, (e.g., about 3× or more, about 4× or more, about 5× or more,about 6× or more, about 7× or more, about 8× or more, about 9× or more,about 10× or more). More generally, however, projection objectives canbe designed to provide a magnified image or an image the same size asthe object.

Referring also to FIG. 2, rays 152 define a cone of light paths thatform the reticle image at image plane 102. The angle of the cone of raysis related to the image-side numerical aperture (NA) of projectionobjective 101. Image-side NA can be expressed as

NA=n_(o) sin θ_(max),

where no refers to the refractive index of the immersing medium adjacentthe surface of substrate 150 (e.g., air, nitrogen, water, or evacuatedenvironment), and θ_(max) is the half-angle of the maximum cone of imageforming rays from projection objective 101.

In general, projection objective 101 can have an image side NA of about0.1 or more (e.g., about 0.15 or more, about 0.2 or more, about 0.25 ormore, about 0.28 or more, about 0.3 or more, about 0.35 or more). Insome embodiments, projection objective 101 has a relatively highimage-side NA. For example, in some embodiments, projection objective101 has an image-side NA of more than 0.4 (e.g., about 0.45 or more,about 0.5 or more, about 0.55 or more, about 0.6 or more). In general,the resolution of projection objective 101 varies depending onwavelength λ and the image-side NA. Without wishing to be bound bytheory, the resolution of a projection objective can be determined basedon the wavelength and image-side NA based on the formula,

${R = {k\frac{\lambda}{N\; A}}},$

where R is the minimum dimension that can be printed and k is adimensionless constant called the process factor. k varies depending onvarious factors associated with the radiation (e.g., the polarizationproperties), the illumination properties (e.g., partial coherence,annular illumination, dipole settings, quadrupole settings, etc.) andthe resist material. Typically, k is in a range from about 0.4 to about0.8, but can also be below 0.4 and higher than 0.8 for certainapplications.

Projection objective 101 is also nominally telecentric at the imageplane. For example, the chief rays can deviate by about 0.5° or less(e.g., about 0.4° or less, about 0.3° or less, about 0.2° or less, about0.1° or less, about 0.05° or less, 0.01° or less, 0.001° or less) frombeing parallel to each other at the image plane over the exposed field.Thus, projection objective 101 can provide substantially constantmagnification over a range of image-size working distances. In someembodiments, the chief rays are nominally orthogonal to image plane 102.Thus, a non flat topography of the wafer surface or defocusing does notlead necessarily to distortion or shading effects in the image plane.

In certain embodiments, projection objective 101 has a relatively highresolution (i.e., the value of R can be relatively small). For example,R can be about 150 nm or less (e.g., about 130 nm or less, about 100 nmor less, about 75 nm or less, about 50 nm or less, about 40 nm or less,about 35 nm or less, about 32 nm or less, about 30 nm or less, about 28nm or less, about 25 nm or less, about 22 nm or less, about 20 nm orless, about 18 nm or less, about 17 nm or less, about 16 nm or less,about 15 nm or less, about 14 nm or less, about 13 nm or less, about 12nm or less, about 11 nm or less, such as about 10 nm).

The quality of images formed by projection objective 101 can bequantified in a variety of different ways. For example, images can becharacterized based on the measured or calculated departures of theimage from idealized conditions associated with Gaussian optics. Thesedepartures are generally known as aberrations. One metric used toquantify the deviation of a wavefront from the ideal or desired shape isthe root-mean-square wavefront error (W_(rms)). W_(rms) is defined inthe “Handbook of Optics,” Vol. I, 2^(nd) Ed., edited by Michael Bass(McGraw-Hill, Inc., 1995), at page 35.3, which is incorporated herein byreference. In general, the lower the W_(rms) value for an objective, theless the wavefront deviates from its desired or ideal shape, and thebetter the quality of the image. In certain embodiments, projectionobjective 101 can have a relatively small W_(rms) for images at imageplane 102. For example, projection objective 101 can have a W_(rms) ofabout 0.1λ or less (e.g., about 0.07λ or less, about 0.06λ or less,about 0.05λ or less, about 0.045λ or less, about 0.04λ or less, about0.035λ or less, about 0.03λ or less, about 0.025λ or less, about 0.02λor less, about 0.015λ or less, about 0.01λ or less, such as about0.005λ).

Another metric that can be used to evaluate the quality of the image isreferred to as field curvature. Field curvature refers to thepeak-to-valley distance for the field point dependent position of thefocal plane. In some embodiments, projection objective 101 can have arelatively small field curvature for images at image plane 102. Forexample, projection objective 101 can have an image-side field curvatureof about 50 nm or less (e.g., about 30 nm or less, about 20 nm or less,about 15 nm or less, about 12 nm or less, 10 nm or less).

A further metric that can be used to evaluate the optical performance isreferred to as distortion. Distortion refers to the maximum absolutevalue of the field point dependent deviation from the ideal image pointposition in the image plane. In some embodiments, projection objective101 can have a relatively small maximum distortion. For example,projection objective 101 can have a maximum distortion of about 50 nm orless, (e.g. about 40 nm or less, about 30 nm or less, about 20 nm orless, about 15 nm or less, about 12 nm or less, 10 nm or less, 9 nm orless, 8 nm or less, 7 nm or less, 6 nm or less, 5 nm or less, 4 nm orless, 3 nm or less, 2 nm or less, such as 1 nm).

Further, in certain embodiments, distortion can vary by a relativelysmall amount across the image field. For example, distortion can vary byabout 5 nm or less (e.g., about 4 nm or less, about 3 nm or less, about2 nm or less, about 1 nm or less) across the image field.

Being a catoptric system, projection objective 101 includes a number ofmirrors arranged to direct radiation reflected from reticle 140 tosubstrate 150 in a way that forms an image of reticle 140 on the surfaceof substrate 150. Specific designs of projection objectives aredescribed below. More generally, however, the number, size, andstructure of the mirrors generally depends on the desired opticalproperties of projection objective 101 and the physical constraints oftool 100.

In general, the number of mirrors in projection objective 101 may vary.Typically, the number of mirrors is related to various performancetrade-offs associated with the optical performance characteristics ofthe objective, such as the desired throughput (e.g., the intensity ofradiation from the object that forms the image at image plane 102), thedesired image-side NA and related image resolution, and the desiredmaximum pupil obscuration.

In general, projection objective 101 has at least four mirrors (e.g.,five or more mirrors, six or more mirrors, seven or more mirrors, eightor more mirrors, nine or more mirrors, ten or more mirrors, eleven ormore mirrors, twelve or more mirrors). In embodiments where it isdesirable that all the mirrors of the objective are positioned betweenthe object plane and the image plane, objective 101 will typically havean even number of mirrors (e.g., four mirrors, six mirrors, eightmirrors, ten mirrors). In certain embodiments, an odd number of mirrorscan be used where all the mirrors of the projection objective arepositioned between the object plane and image plane. For example, whereone or more mirrors are tilted at relatively large angles, a projectionobjective can include an odd number of mirrors where all the mirrors arepositioned between the object plane and image plane.

In general, at least one of the mirrors in projection objective 101 hasa freeform surface. Unlike spherical or aspherical mirrors, freeformmirror surfaces do not have an axis of rotational symmetry. Generally, afreeform surface deviates from a best fit rotationally symmetric surface(e.g., a spherical or aspherical surface). Rotationally-symmetricreference surfaces can be determined for a freeform mirror surface asfollows. First, one obtains information that characterizes the freeformmirror surface under consideration. In embodiments where optical data ofthe mirror is known, this information includes determining the basicradius of the mirror (e.g. 1/c, where c is the vertex curvature), aconic constant of the mirror, k, and polynomial coefficientscharacterizing the mirror. Alternatively, or additionally, theinformation characterizing the mirror can be obtained from a surfacefigure measurement of the mirror surface (e.g. obtained using aninterferometer). A surface figure measurement can provide a functionz′(x′, y′) describing the mirror's surface, where z′ is the sag of themirror surface along the z′-axis for different (x′, y′) coordinates, asillustrated in FIG. 2B. The initial step also includes determining thefootprint for the mirror, which refers to an area of the mirror surfacethat is actually used to reflect image-forming radiation in theobjective. The footprint can be determined by tracing rays through theobjective using a ray tracing program and extracting the mirror areacontacted by the rays.

After obtaining the information characterizing the rotationallyasymmetric surface, a local coordinate system for the surface isestablished for which decentration and tilt of the surface is zero.Setting the tilt and decentration of the surface gives a well definedstarting point for an optimization algorithm to determine the referencesurface and also define an axis, z′, along which the sag differencesbetween the mirror surface and the reference surface can be determined.Where optical data for the mirror surface is known, the z′-axis isdetermined based on the conic constant, k, and basic radius, 1/c. Forthe rotationally symmetric portion of the optical data, the z′-axis isthe symmetry axis for the rotationally symmetric part of therotationally asymmetric surface. In embodiments where the mirror surfaceis characterized from a surface figure measurement, the z′-axiscorresponds to the metrology axis (e.g. the interferometers opticalaxis). FIG. 2B illustrates this for a two-dimensional section of arotationally asymmetric mirror 201, where the local coordinate system isdenoted by the x′, y′ and z′ axes. The boundaries for the footprint ofthe rotationally asymmetric mirror 201 are shown as x_(min) and x_(max)for the cross-section shown in FIG. 2B.

An initial reference surface is then established with respect to thecoordinate system. The initial reference surface has zero tilt and zerodecentration. The initial reference surface is either a sphericalsurface or a rotationally symmetric aspherical surface. The initialreference surface is established by one designating a rotationallysymmetric surface that approximates the rotationally asymmetric mirrorsurface. The initial reference surface represents a starting point foran optimization algorithm. Once the initial reference surface isestablished, a local distance, b_(i) (i=1 . . . N) between a number ofpoints of the initial reference surface and points on the surface of therotationally asymmetric surface footprint measured along the z′-axis ofthe local coordinate system are determined. Next, the rotationallysymmetric reference surface (surface 211 in FIG. 2B) is established bydetermining a minimal value for the local distances (d_(i)) using anumber fitting parameters and a fitting algorithm. In the event that therotationally symmetric reference surface is a spherical surface, theparameters include the location of the center of the sphere within thelocal coordinate system, the radius, of the reference surface. In FIG.2B, decentering of the sphere center from the coordinate system originis shown by coordinates x_(c) and z_(c) (decentration along the y′-axisby an amount y_(c) is not shown in FIG. 2B). The radius of the sphericalsurface is designated as R. The parameters R, x_(c), y_(c) and z_(c) areoptimized to provide a minimal value for the local distances, d_(i),based on the equation:

z′=(R ²−(x′−x _(c))²−(y′−y _(c))²)^(1/2) −z _(c),

which is the equation for a spherical surface of radius R, centered atcoordinate (x_(c), y_(c), z_(c)).

Where the rotationally symmetric reference surface is an asphericalsurface, the parameters can include decentration and tilt of thereference surface, base radius, conical constant and asphericalcoefficients. These parameters can be determined based on the equation

${z^{\prime} = {\frac{c^{\prime}h^{2}}{1 + \sqrt{1 - {( {1 + k^{\prime}} )c^{\prime 2}h^{2}}}} + {\sum\limits_{j}{A_{j}^{\prime}h^{2j}}}}},$

which is an equation describing conic and aspheric surfaces. Here,h²=x′²+y′², and A′_(j) are coefficients characterizing the deviation ofthe rotationally-symmetric reference surface from a conic surface.Generally, the number of aspherical coefficients, A′_(j), used to fitthe reference surface to the mirror surface can vary depending on thecomputational power of the system being used to calculate the surface,the time available, and the desired level of accuracy. In someembodiments, the reference surface can be calculated using asphericalcoefficients up to third order. In certain embodiments, coefficientshigher than third order (e.g., fourth order, sixth order) are used. Foradditional discussion on parameterization of conic and aspheric surfacessee, for example, the product manual for Code V, available from OpticalResearch Associates (Pasadena, Calif.).

In general, fitting can be performed using a variety of optimizationalgorithms. For example, in some embodiments, a least squares fittingalgorithm, such as a damped least squares fitting algorithm, can beused. Damped least squares fitting algorithms may be performed usingcommercially-available optical design software, such as Code V or ZEMAX(available from Optima Research, Ltd., Stansted, United Kingdom) forexample.

After the rotationally-symmetric reference surface is determined, thelocal distance between additional points on the mirror surface can bedetermined and visualized. Additional characteristics of therotationally-symmetric reference surface can be determined. For example,a maximum deviation of the rotationally-symmetric reference surface fromthe rotationally-asymmetric mirror surface can be determined.

A freeform surface can, for example, have a maximum deviation from abest fit sphere of about λ or more (e.g., about 10λ or more, about 20λor more, about 50λ or more, about 100λ or more, about 150λ or more,about 200λ or more, about 500λ or more, about 1,000λ or more, about10,000λ or more, about 50,000λ or more). A freeform surface can have amaximum deviation from a best fit rotationally symmetric asphere ofabout λ or more (e.g., about 5λ or more, about 10λ or more, about 20λ ormore, about 50λ or more, about 100λ or more, about 200λ or more, about500λ or more, about 1,000λ or more, about 10,000λ or more). In someembodiments, a freeform surface can have a maximum deviation from a bestfit rotationally symmetric asphere of about 1,000λ or less (e.g., about900λ or less, about 800λ or less, about 700λ or less, about 600λ orless, about 500λ or less).

In certain embodiments, freeform surfaces have a maximum deviation froma best fit sphere by 10 nm or more (e.g., about 100 nm or more, about500 nm or more, about 1 μm or more, about 5 μm or more, about 10 μm ormore, about 50 μm or more, about 100 μm or more, about 200 μm or more,about 500 μm or more, about 1,000 μm, about 2,000 μm or more, about3,000 μm or more). Freeform surfaces can have a maximum deviation from abest fit sphere by about 10 mm or less (e.g., about 5 mm or less, about3 mm or less, about 2 mm or less, about 1 mm or less, about 500 μm orless).

Freeform surfaces can have a maximum deviation from a best fitrotationally symmetric asphere by 10 nm or more (e.g., about 100 nm ormore, about 500 nm or more, about 1 μm or more, about 5 μm or more,about 10 μm or more, about 50 μm or more, about 100 μm or more, about200 μm or more, about 500 μm or more, about 1,000 μm). Freeform surfacescan have a maximum deviation from a best fit rotationally symmetricasphere by about 10 mm or less (e.g., about 5 mm or less, about 3 mm orless, about 2 mm or less, about 1 mm or less, about 500 μm or less).

The curvature of the mirror surfaces is characterized by a first andsecond mean principal curvature, which are determined at the point oneach mirror surface that reflects the chief ray of the central fieldpoint. First and second principal curvatures are calculated as describedin Handbook of Mathematics by I. N. Bronstein, et al., 4^(th) Ed.(Springer, 2004), p. 567. In general, the first principal curvature fora mirror can be different from the second principal curvature for thatmirror. In some embodiments, the absolute value of the differencebetween the first and second principal curvatures can be about 10⁻⁸ ormore (e.g., 10⁻⁷ or more, 5×10⁻⁷ or more, about 10⁻⁶ or more, about5×10⁻⁶ or more, about 10⁻⁵ or more, about 5×10⁻⁵ or more, about 10⁻⁴ ormore, about 5×10⁻⁴ or more, about 10⁻³ or more).

In general, the first and/or second principal curvatures can be positiveor negative. The first and/or second principal curvatures for a mirrorsurface can be relatively small. For example, in some embodiments, theabsolute value of the first principal curvature for one or more mirrorsin projection objective 101 is about 10⁻² or less (e.g., about 5×10⁻³ orless, about 3×10⁻³ or less, about 2×10⁻³ or less, about 10⁻³ or less).The absolute value of the sum of the first principal curvatures for themirrors in projective objective 101 can be about 10⁻³ or less (e.g.,about 5×10⁻⁴ or less, about 3×10⁻⁴, about 2×10⁻⁴ or less, about 10⁻⁴ orless, 5×10⁻⁵ or less, 10⁻⁵ or less).

In certain embodiments, the absolute value of the second principalcurvature for one or more mirrors in projection objective 101 is about10⁻² or less (e.g., about 5×10⁻³ or less, about 3×10⁻³ or less, about2×10⁻³ or less, about 10⁻³ or less). The absolute value of the sum ofthe second principal curvatures for the mirrors in projective objective101 can be about 10⁻³ or less (e.g., about 5×10⁻⁴ or less, about 3×10⁻⁴,about 2×10⁻⁴ or less, about 10⁻⁴ or less, 5×10⁻⁵ or less, 10⁻⁵ or less).

The sum of the first and second principal curvatures of the mirrors inprojection objective 101 can be relatively small. For example, theabsolute value of the sum of the first and second principal curvaturesof the mirrors can be about 10⁻³ or less (e.g., about 5×10⁻⁴ or less,about 3×10⁻⁴, about 2×10⁻⁴ or less, about 10⁻⁴ or less, 5×10⁻⁵ or less,10⁻⁵ or less).

In certain embodiments, freeform mirror surfaces can be describedmathematically by the equation:

$Z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {( {1 + k} )c^{2}r^{2}}}} + {\sum\limits_{j = 2}^{66}{C_{j}X^{m}Y^{n}}}}$where $j = {\frac{( {m + n} )^{2} + m + {3n}}{2} + 1}$

and Z is the sag of the surface parallel to a Z-axis (which may or maynot be parallel to the reference axis 105 in projection objective 101,i.e. in general is decentered and tilted to the reference axis 105 inprojection objective 101), c is a constant corresponding to the vertexcurvature, k is a conic constant, and C_(j) is the coefficient of themonomial X^(m)Y^(n). Typically, the values of c, k, and C_(j) aredetermined based on the desired optical properties of the mirror withrespect to projection objective 101. Further, the order of the monomial,m+n, can vary as desired. Generally, a higher order monomial can providea projection objective design with a higher level of aberrationcorrection, however, higher order monomials are typically morecomputationally expensive to determine. In some embodiments, m+n is 10or more (e.g., 15 or more, 20 or more). As discussed below, theparameters for the freeform mirror equation can be determined usingcommercially-available optical design software. In some embodiments, m+nis less than 10 (e.g., 9 or less, 8 or less, 7 or less, 6 or less, 5 orless, 4 or less, 3 or less).

In general, freeform surfaces can be described mathematically usingequations other than those presented above. For example, in someembodiments, freeform surfaces can be described mathematically usingZernike polynomials (such as presented in the manual for CODE V®,commercially available from Optical Research Associates, Pasadena,Calif.) or using two-dimensional spline surfaces. Examples oftwo-dimensional spline surfaces are Bezier splines or non-uniformrational Bezier splines (NURBS). Two-dimensional spline surfaces can bedescribed, for example, by a grid of points in an x-y plane andcorresponding z-values or slopes and these points. Depending on thespecific type of spline surface, the complete surface is obtained by aspecific interpolation between the grid points using, e.g., polynomialsor functions that have certain properties with respect to continuity ordifferentiability (e.g., analytical functions).

In general, the number and position of freeform mirrors in projectionobjective 101 can vary. Embodiments include projection objectives withtwo or more freeform mirrors (e.g., three or more freeform mirrors, fouror more freeform mirrors, five or more free-form mirrors, six or morefreeform mirrors).

Projection objective 101 generally includes one or more mirrors withpositive optical power. In other words, the reflective portion of themirror has a concave surface and is referred to as a concave mirror.Projection objective 101 can include two or more (e.g., three or more,four or more, five or more, six or more) concave mirrors. Projectionobjective 101 can also include one or more mirrors that have negativeoptical power. This means that one or more of the mirrors has areflective portion with a convex surface (referred to as a convexmirror). In some embodiments, projection objective 101 includes two ormore (e.g., three or more, four or more, five or more, six or more)convex mirrors.

Referring to FIG. 10, an embodiment of a projection objective 1100includes six mirrors 1110, 1120, 1130, 1140, 1150, and 1160, and has animage-side numerical aperture of 0.35 and an operating wavelength of13.5 nm. Mirrors 1110, 1120, 1130, 1140, 1150, and 1160 are all freeformmirrors. Data for projection objective 1100 is presented in Table 1A andTable 1B below. Table 1A presents optical data, while Table 1B presentsfree-form constants for each of the mirror surfaces. For the purposes ofTable 1A and Table 1B, the mirror designations correlate as follows:mirror 1 (M1) corresponds to mirror 1110; mirror 2 (M2) corresponds tomirror 1120; mirror 3 (M3) corresponds to mirror 1130; mirror 4 (M4)corresponds to mirror 1140; mirror 5 (M5) corresponds to mirror 1150;and mirror 6 (M6) corresponds to mirror 1160. “Thickness” in Table 1Aand subsequent tables refers to the distance between adjacent elementsin the radiation path. The monomial coefficients, C_(j), for thefreeform mirrors, along with the amount the mirror is decentered androtated (or tilted) from an initial projection objective design, areprovided in Table 1B. R, the radius, is the inverse of the vertexcurvature, c. Decenter is given in mm and rotation is given in degrees.Units for the monomial coefficients are mm^(−j+1). Nradius is a unitlessscaling factor (see, for example, the manual for CODE V®).

In FIG. 10, projection objective 1100 is shown in meridional section.The meridional plane is a symmetry plane for projection objective 1100.Symmetry about the meridional plane is as the mirrors are decenteredonly with respect to the y-axis and tilted about the x-axis. Further,the coefficients for the freeform mirrors having an odd degree in thexcoordinate (e.g., x, x³, x⁵, etc.) are zero.

Projection objective 1100 images radiation from object plane 103 toimage plane 102 with a demagnification ratio of 4×. The tracklength ofprojection objective 1100 is 2000 mm and the optical path length ofimaged radiation is 5337 mm. Accordingly, the ratio of the optical pathlength to tracklength is approximately 2.67. Projection objective 1100has an aperture stop 1106 positioned at mirror 1120.

The entrance pupil of projection objective 1100 is located at infinity.The chief ray angle of the central field point at object plane 103 is7°. The maximum variation of chief ray angles at object plane 103 isless than 0.06°. Projection objective 1100 is telecentric on the objectside.

Projection objective 1100 has a rectangular field. The image-side fieldwidth, d_(x), is 26 mm. The image-side field length, d_(y), is 2 mm.Projection objective 1100 has an object-image shift of 31 mm.

The performance of projection objective 1100 includes an image-sideW_(rms) of 0.025λ. Image-side field curvature is 10 nm. Projectionobjective 1100 provides an intermediate image between mirrors 1140 and1150.

The optical power of the mirrors in the order of the radiation path fromobject plane 103 to image plane 102 is as follows: mirror 1110 haspositive optical power; mirror 1120 has positive optical power; mirror1130 has negative optical power; mirror 1140 has positive optical power;mirror 1150 has negative optical power; and mirror 1160 has positiveoptical power.

The dimension of the footprint of each mirror, given as M_(x)×M_(y), isas follows: 291 mm×195 mm for mirror 1110; 159 mm×152 mm for mirror1120; 157 mm×53 mm for mirror 1130; 295 mm×66 mm for mirror 1140; 105mm×86 mm for mirror 1150; and 345 mm×318 mm for mirror 1160.

The chief ray angle of incidence for the central field point is 4.38°,4.03°, 18.37°, 7.74°, 12.64°, and 5.17° for mirrors 1110, 1120, 1130,1140, 1150, and 1160, respectively. The maximum angle of incidence,θ_(max), on each mirror for the meridional section is 6.48°, 6.44°,20.05°, 9.12°, 21.76°, and 7.22° for mirrors 1110, 1120, 1130, 1140,1150, and 1160, respectively. Δθ for mirrors 1110, 1120, 1130, 1140,1150, and 1160 are 4.27°, 4.92°, 4.09°, 3.12°, 19.37°, and 4.61°,respectively.

Mirrors 1110, 1150, and 1160 have freeboards that are more than 5 mm andless than 25 mm. Mirror 1140 has positive chief ray angle magnificationwhile mirrors 1110, 1120, 1130, and 1150 have negative chief ray anglemagnification.

The image-side free working distance of projection objective 1100 is 25mm. The object-side free working distance is 163 mm.

In projection objective 1100, d_(op-1)/d_(op-2) is 6.57. Furthermore,adjacent mirror pair 1040 and 1050 are separated by more than 50% of theprojection objective's tracklength. Further, the distance between mirror1110 and object plane 103 is more than 50% of the projection objective'stracklength.

Data for projection objective 1100 is presented in Table 1A and Table 1Bbelow. Table 1A presents optical data, while Table 1B presentsaspherical constants for each of the mirror surfaces. For the purposesof Table 1A and Table 1B, the mirror designations correlate as follows:mirror 1 (M1) corresponds to mirror 1110; mirror 2 (M2) corresponds tomirror 1120; mirror 3 (M3) corresponds to mirror 1130; mirror 4 (M4)corresponds to mirror 1140; mirror 5 (M5) corresponds to mirror 1150;and mirror 6 (M6) corresponds to mirror 1160.

TABLE 1A Surface Radius (mm) Thickness (mm) Mode Object INFINITY1070.002 Mirror 1 −2069.710 −907.121 REFL Mirror 2 1710.596 0.000 REFLSTOP INFINITY 907.500 Mirror 3 414.111 −319.107 REFL Mirror 4 618.0221223.709 REFL Mirror 5 406.139 −436.552 REFL Mirror 6 522.609 461.570REFL Image INFINITY 0.000

TABLE 1B Coefficient M1 M2 M3 M4 M5 M6 K −2.012543E+00 −7.790981E+00  −9.061196E−01 −4.714699E−01   5.253415E+00 1.051556E−01 Y −1.801229E−01−2.676895E−01     6.249715E−03 2.914352E−02 3.699848E−02 6.762162E−04 X²−3.718177E−05 −1.568640E−04   −4.213586E−04 −1.680785E−04  −6.132874E−05   2.479745E−06 Y² −5.757281E−05 −1.359112E−04  −3.015850E−04 −9.908817E−05   −6.383717E−05   1.909227E−06 X²Y−3.283304E−08 −1.421641E−07   −4.802304E−08 −4.234719E−08   5.460366E−07−5.398408E−09   Y³ −7.289267E−08 −9.447144E−08     3.714670E−071.405667E−07 2.644773E−08 −4.741511E−09   X⁴ −3.792148E−11 2.173390E−10−8.723035E−10 −2.377992E−11   1.030821E−09 −1.926536E−11   X²Y²−1.087876E−10 5.689855E−10 −5.959943E−10 −4.401654E−10   2.045233E−09−4.586698E−11   Y⁴ −1.237594E−10 2.990476E−10   8.549602E−10−4.022663E−11   5.551510E−11 −2.632066E−11   X⁴Y −3.587007E−14−1.028868E−12   −8.033093E−12 1.716353E−13 5.551826E−12 −2.577816E−14  X²Y³   8.925822E−14 4.492952E−13 −1.186636E−12 −7.545064E−13  −4.309344E−12   −1.775797E−14   Y⁵ −7.423435E−14 5.791519E−13  8.705928E−14 −2.700779E−13   −7.302230E−12   −9.309635E−15   X⁶  1.876383E−17 2.916278E−16 −2.307341E−14 −1.670466E−15   8.878140E−15−3.351380E−17   X⁴Y² −3.009967E−16 −3.620666E−16   −2.232847E−141.589023E−15 4.463758E−14 −1.408427E−16   X²Y⁴   1.992400E−163.916129E−16   1.756497E−15 3.477633E−16 1.478648E−13 −1.372823E−16   Y⁶  8.315953E−18 −6.580116E−16     8.232062E−16 1.253553E−16 3.691569E−14−3.799352E−17   X⁶Y −2.621825E−20 −1.237101E−17   −3.125465E−16−7.682746E−18   3.293829E−16 −1.214309E−19   X⁴Y³ −1.344388E−183.730815E−17   1.376670E−16 5.918289E−18 8.409538E−16 5.369262E−20 X²Y⁵−6.157858E−19 3.202677E−17   4.387074E−19 2.707480E−18 4.875870E−16−1.363873E−20   Y⁷   2.770009E−20 8.487049E−18   2.518948E−181.820744E−19 1.274511E−16 2.776746E−21 X⁸   2.265356E−23 −1.881878E−20    6.916970E−19 3.815768E−20 −1.030207E−19   −2.085793E−23   X⁶Y²−1.848041E−22 −1.667898E−19   −1.070800E−18 1.947584E−20 −6.071205E−19  −1.191227E−22   X⁴Y⁴ −1.617091E−21 −4.471313E−20   −2.039154E−19−1.469302E−21   8.581801E−18 −2.848570E−22   X²Y⁶ −1.152811E−21−1.417078E−19   −4.885470E−20 8.329380E−22 2.867618E−18 8.073429E−24 Y⁸  5.021474E−23 −1.270497E−20   −2.834042E−20 −1.011971E−21  1.813992E−18 −6.757839E−23   X⁸Y   0.000000E+00 0.000000E+00  7.973679E−21 2.492982E−22 0.000000E+00 −2.465296E−25   X⁶Y³  0.000000E+00 0.000000E+00   7.629111E−22 1.401277E−22 0.000000E+002.930653E−25 X⁴Y5   0.000000E+00 0.000000E+00 −7.196032E−21−4.219890E−23   0.000000E+00 1.194933E−25 X²Y⁷   0.000000E+000.000000E+00 −1.090375E−22 −3.791571E−24   0.000000E+00 5.412579E−25 Y⁹  0.000000E+00 0.000000E+00 −5.080252E−23 1.076602E−24 0.000000E+003.891280E−26 X¹⁰   0.000000E+00 0.000000E+00 −6.129418E−25−1.289913E−27   0.000000E+00 0.000000E+00 X⁸Y²   0.000000E+000.000000E+00   2.295090E−23 4.078311E−25 0.000000E+00 0.000000E+00 X⁶Y⁴  0.000000E+00 0.000000E+00   5.951785E−24 1.728297E−25 0.000000E+000.000000E+00 X⁴Y⁶   0.000000E+00 0.000000E+00 −1.732732E−23−5.280557E−26   0.000000E+00 0.000000E+00 X²Y⁸   0.000000E+000.000000E+00   0.000000E+00 −1.410994E−27   0.000000E+00 0.000000E+00Y¹⁰   0.000000E+00 0.000000E+00   0.000000E+00 3.484416E−27 0.000000E+000.000000E+00 Nradius   1.000000E+00 1.000000E+00   1.000000E+001.000000E+00 1.000000E+00 1.000000E+00 Y-decenter 194.936 −49.734 36.6099.442 30.019 40.956 X-rotation −5.944 −17.277 −5.569 −0.579 0.301 −0.924

In certain embodiments, the arrangement of mirrors in projectionobjective 101 images radiation from object plane 103 to one or moreintermediate-image planes. Embodiments that have one or moreintermediate images, also include two or more pupil planes. In someembodiments, at least one of these pupil planes is physically accessiblefor the purposes of placing an aperture stop substantially at that pupilplane. An aperture stop is used to define the size of the projectionobjective's aperture.

Coma at an intermediate image in projection objective 101 can berelatively large. Coma can be quantified by the distance between thechief ray and the upper and lower rays at the point where the upper andlower rays cross. In some embodiments, this distance can be about 1 mmor more (e.g., about 2 mm or more, about 3 mm or more, about 4 mm ormore, about 5 mm or more, about 6 mm or more, such as about 7 mm). Comaat an intermediate image in projection objective can be relativelysmall. In some embodiments, the distance can be about 1 mm or less(e.g., about 0.1 mm or less, 0.01 mm or less).

In general, mirrors in projection objective 101 are formed so that theyreflect a substantial amount of radiation of wavelength λnormally-incident thereon or incident thereon over a certain range ofincident angles. Mirrors can be formed, for example, so that theyreflect about 50% or more (e.g., about 60% or more, about 70% or more,about 80% or more, about 90% or more, about 95% or more, 98% or more) ofnormally incident radiation at λ.

In some embodiments, the mirrors include a multilayer stack of films ofdifferent materials arranged to substantially reflect normally incidentradiation at λ. Each film in the stack can have an optical thickness ofabout λ/4. Multilayer stacks can include about 20 or more (e.g., about30 or more, about 40 or more, about 50 or more) films. In general, thematerials used to form the multilayer stacks are selected based onoperational wavelength λ. For example, multiple alternating films ofmolybdenum and silicon or molybdenum and beryllium can be used to formmirrors for reflecting radiation in the 10 nm to 30 nm range (e.g., forλ of about 13 nm or about 11 nm, respectively). Generally, multiplealternating films of molybdenum and silicon can be used for λ=11 nm andmultiple alternating films of molybdenum and beryllium can be used forλ=13 nm.

In certain embodiments, the mirrors are made of quartz glass coated witha single layer of aluminum and overcoated with one or more layers ofdielectric materials, such as layers formed from MgF₂, LaF₂, or, Al₂O₃Mirrors formed from aluminum with dielectric coatings can be used, forexample, for radiation having a wavelength of about 193 nm.

In general, the percentage of radiation at λ reflected by a mirrorvaries as a function of the angle of incidence of the radiation on themirror surface. Because imaged radiation propagates through a catoptricprojection objective along a number of different paths, the angle ofincidence of the radiation on each mirror can vary. This effect isillustrated with reference to FIG. 3, which shows a portion of a mirror400, in meridional section, that includes a concave reflective surface401. Imaged radiation is incident on surface 401 along a number ofdifferent paths, including the paths shown by rays 410, 420, and 430.Rays 410, 420, and 430 are incident on portions of surface 401 where thesurface normal is different. The direction of surface normal at theseportions is shown by lines 411, 421, and 431, corresponding to rays 410,420, and 430, respectively. Rays 410, 420, and 430 are incident onsurface 401 at angles θ₄₁₀, θ₄₂₀, and θ₄₃₀, respectively. In general,angles θ₄₁₀, θ₄₂₀, and θ₄₃₀ may vary.

For each mirror in projection objective 101, the incident angles ofimaged radiation can be characterized in a variety of ways. Onecharacterization is the maximum angle of incidence of meridional rays oneach mirror in a meridional section of projection objective 101.Meridional rays refer to rays lying in the meridional section. Ingeneral, θ_(max) can vary for different mirrors in projection objective101.

In some embodiments, the maximum value of θ_(max) for all the mirrors inprojection objective 101 is about 75° or less (e.g., about 70° or less,about 65° or less, about 60° or less, about 55° or less, about 50° orless, about 45° or less). θ_(max) can be more than about 5° (e.g., about10° or more, about 20° or more). In some embodiments, the maximum valueof θ_(max) can be relatively low. For example, the maximum value ofθ_(max) can be about 40° or less (e.g., about 35° or less, about 30° orless, about 25° or less, about 20° or less, about 15° or less, about 13°or less, about 10° or less).

In some embodiments, the ratio of the maximum value of θ_(max) (indegrees) to image-side NA can be about 10° or less (e.g., about 8° orless, about 7° or less, 68 or less, about 60 or less, about 5° or less,about 4° or less, about 3° or less).

Another characterization is the angle of incidence of the chief raycorresponding to the central field point on each mirror in a meridionalsection of projection objective 101. This angle is referred to asθ_(CR). In general, θ_(CR) can vary. In some embodiments the maximumvalue of θdCR, θ_(CR)(max), in projection objective 101 can berelatively low. For example, θ_(CR)(max) can be about 35° or less (e.g.,about 30° or less, about 25° or less, about 20° or less, about 15° orless, about 13° or less, about 10° or less, about 8° or less, about 5°or less).

In some embodiments, the ratio of the maximum value of θ_(CR(max)) (indegrees) to image-side NA can be about 10° or less (e.g., about 8° orless, about 7° or less, 68 or less, about 6° or less, about 5° or less,about 4° or less, about 3° or less).

Each mirror in projection objective 101 can also be characterized by therange of angles of incidence, Δθ, of rays for a meridional section ofprojection objective 101. For each mirror, Δθ corresponds to thedifference between θ_(max) and θ_(min), where θ_(min) is the minimumangle of incidence of rays on each mirror in a meridional section ofprojection objective 101. In general, Δθ may vary for each mirror inprojection objective 101. For some mirrors, Δθ can be relatively small.For example, Δθ can be about 20° or less (e.g., about 15° or less, about12° or less, about 10° or less, about 8° or less, about 5° or less,about 3° or less, 2° or less). Alternatively, for some mirrors inprojection objective 101, Δθ can be relatively large. For example, Δθfor some mirrors can be about 20° or more (e.g., about 25° or more,about 30° or more, about 35° or more, about 40° or more).

In some embodiments, the maximum value for Δθ, Δθ_(max), for all themirrors in projection objective 101 can be relatively small. Forexample, Δθ_(max) can be about 25° or less (e.g., about 20° or less,about 15° or less, about 12° or less, about 10° or less, about 9° orless, about 8° or less, about 7° or less, about 6° or less, about 5° orless, such as 3°).

Another way to characterize the radiation path in projection objective101 is by the chief ray magnification at each mirror, which refers tothe quotient of the tangent of the angle between the chief ray (e.g. inthe meridional section) and reference axis 105 before and afterreflection from each mirror. For example, referring to FIG. 4 where achief ray 501 diverges from reference axis 105 prior to reflection froma mirror 510, and reflects from mirror 510 back towards reference axis105, mirror 510 has a positive chief ray angle magnification. Referringto FIG. 5, alternatively, where a chief ray 502 diverges from referenceaxis 105 both before and after reflection from a mirror 520, mirror 520has a negative chief ray angle magnification. In both cases, the chiefray magnification is given by tan(α)/tan(β). In certain embodiments,having multiple mirrors with positive chief ray angle magnification cancorrespond to relatively large incident angles on one or more mirrors inthe projection objective. Accordingly, projection objectives having onlyone mirror with positive chief ray angle magnification can also exhibitrelatively low incident ray angles on the mirrors.

The relative spacing of mirrors in projection objective 101 can varydepending on the specific design of the projection objective. Relativelylarge distances between adjacent mirrors (with respect to the path ofthe radiation) can correspond to relatively low incident ray angles onthe mirrors. In certain embodiments, projection objective 101 caninclude at least one pair of adjacent mirrors that separated by morethan 50% of the projection objective tracklength.

In certain embodiments, having a large relative distance, d_(op−1),between the object plane and the first mirror in the radiation pathcompared to the distance, d_(op−2), between the object plane and thesecond mirror in the radiation path can also correspond to relativelylow incident ray angles on the mirrors. For example, embodiments whered_(op−1)/d_(op−2) is about 2 or more (e.g., about 2.5 or more, about 3or more, about 3.5 or more, about 4 or more, about 4.5 or more, about 5or more) can also have relatively low incident ray angles.

In general, the footprint size and shape of the mirrors in projectionobjective 101 can vary. The footprint shape refers to the shape of themirror projected onto the x-y plane. The footprint of the mirrors can becircular, oval, polygonal (e.g., rectangular, square, hexagonal), orirregular in shape. In embodiments, the footprint is symmetric withrespect to the meridional plane of projection objective 101.

In certain embodiments, mirrors can have a footprint with a maximumdimension that is about 1,500 mm or less (e.g., about 1,400 nm or less,about 1,300 mm or less, about 1,200 mm or less, about 1,100 mm or less,about 1,000 mm or less, about 900 mm or less, about 800 mm or less,about 700 mm or less, about 600 mm or less, about 500 mm or less, about400 mm or less, about 300 mm or less, about 200 mm or less, about 100 mmor less.) Mirrors may have footprint that has a maximum dimension thatis more than about 10 mm (e.g., about 20 mm or more, about 50 mm ormore).

An example of a mirror 600 with an oval footprint is shown in FIG. 6A.Mirror 600 has a maximum dimension in the x-direction given by M_(x). Inembodiments, M_(x) can be about 1,500 mm or less (e.g., about 1,400 nmor less, about 1,300 mm or less, about 1,200 mm or less, about 1,100 mmor less, about 1,000 mm or less, about 900 mm or less, about 800 mm orless, about 700 mm or less, about 600 mm or less, about 500 mm or less,about 400 mm or less, about 300 mm or less, about 200 mm or less, about100 mm or less). M_(x) can be more than about 10 mm (e.g., about 20 mmor more, about 50 mm or more).

Mirror 600 is symmetric with respect to meridian 601. Mirror 600 has adimension M_(y) along meridian 601. For mirror 600 M_(y) is smaller thanM_(x), however, more generally, M_(y) can be smaller, the same size, orlarger than M_(x). In some embodiments, M_(y) is in a range from about0.1 M_(x) to about M_(x) (e.g., about 0.2 M_(x) or more, about 0.3 M_(x)or more, about 0.4 M_(x) or more, about 0.5 M_(x) or more, about 0.6M_(x) or more, about 0.7 M_(x) or more about 0.8 M_(x) or more, about0.9 M_(x) or more). Alternatively, in certain embodiments, M_(y) can beabout 1.1 M_(x) or more (e.g., about 1.5 M_(x) or more), such as in arange from about 2 M_(x) to about 10 M_(x). M_(y) can be about 1,000 mmor less (e.g., about 900 mm or less, about 800 mm or less, about 700 mmor less, about 600 mm or less, about 500 mm or less, about 400 mm orless, about 300 mm or less, about 200 mm or less, about 100 mm or less).M_(y) can be more than about 10 mm (e.g., about 20 mm or more, about 50mm or more).

In some embodiments, the base of the mirrors may extend beyond themirror surface (i.e., the portion of the mirror that reflects imagedradiation) in one or more directions. For example, a mirror's base canextend about 10 mm or more (e.g., about 20 mm or more, about 30 mm ormore, about 40 mm or more, about 50 mm or more) beyond the opticallyactive surface in the x- and/or y-directions. Mirror base extension canfacilitate mounting the mirror in projection objective 101 by providingsurfaces that are not optically active that can be attached to mountingapparatus.

Optionally, mirror base extensions should not be in a direction thatoccludes the radiation path in projection objective 101. The distancebetween the edge of a mirror and the radiation path as it passes themirror is related to a parameter referred to as the “freeboard,” whichis the minimum distance between the rays closest to a mirror's edge andthe rays nearest the mirror's edge that are reflected by the mirror. Insome embodiments, projection objective 101 can include one or moremirrors with freeboards of about 20 mm or more (e.g., about 25 mm ormore, about 30 mm or more, about 35 mm or more, about 40 mm or more,about 45 mm or more, about 50 mm or more). Large freeboards provideflexibility in mirror fabrication as the projection objective canaccommodate an extended mirror base without occlusion of the imagedradiation. However, relatively small freeboards can correspond to lowincident ray angles on the mirrors in the projection objective. In someembodiments, projection objective 101 can include one or more mirrorswith freeboards of about 15 mm or less (e.g., about 12 mm or less, about10 mm or less, about 8 mm or less, about 5 mm or less). In certainembodiments, projection objective 101 includes one or more mirrorshaving a freeboard between 5 mm and 25 mm.

In general, the thickness of the mirrors in projection objective 101 mayvary. Mirror thickness refers to the dimension of the mirror along thez-axis. Mirrors should generally have sufficient thickness to facilitatemounting within the projection objective. Referring to FIG. 6B, thethickness of mirror 600 can be characterized by a maximum thickness,T_(max), and a minimum thickness, T_(min). Typically, the differencebetween T_(max) and T_(min) will depend on the curvature of the mirrorsurface and the structure of the mirror's base. In some embodiments,T_(max) is about 200 mm or less (e.g., about 150 mm or less, about 100mm or less, about 80 mm or less, about 60 mm or less, about 50 mm orless, about 40 mm or less, about 30 mm or less, about 20 mm or less). Incertain embodiments, T_(min) is about 1 mm or more (e.g., about 2 mm ormore, about 5 mm or more, about 10 mm or more, about 20 mm or more,about 50 mm or more, about 100 mm or more).

In some embodiments, the maximum dimension of any mirror in projectionobjective is about 1,500 mm or less (e.g., about 1,400 nm or less, about1,300 mm or less, about 1,200 mm or less, about 1,100 mm or less, about1,000 mm or less, about 900 mm or less, about 800 mm or less, about 700mm or less, about 600 mm or less, about 500 mm or less, such as about300 mm). In certain embodiments, the maximum dimension of any mirror inprojection objective is about 10 mm or more (e.g., about 20 mm or more,about 30 mm or more, about 40 mm or more, about 50 mm or more, about 75mm or more, about 100 mm or more).

In general, the shape of the field of projection objective 101 can vary.In some embodiments, the field has an arcuate shape, such as the shapeof a segment of a ring. Referring to FIG. 7A, a ring-segment field 700can be characterized by an x-dimension, d_(x), a y-dimension, d_(y), anda radial dimension, d_(r). d_(x) and d_(y) correspond to the dimensionof the field along the x-direction and y-direction, respectively. d_(r)corresponds to the ring radius, as measured from an axis 705 to theinner boundary of field 700. It should be stressed that the axis 705 isno axis to describe the optical system (e.g. it is not the referenceaxis) and in particular is not an optical axis. Axis 705 only serves todefine the ring-segment field 700. Ring-segment field 700 is symmetricwith respect to a plane parallel to the y-z plane and indicated by line710. In general, the sizes of d_(x), d_(y), and d_(r) vary depending onthe design of projection objective 101. Typically d_(y) is smaller thand_(x). The relative sizes of field dimensions d_(x), d_(y), and d_(r) atobject plane 103 and image plane 102 vary depending on the magnificationor demagnification of projection objective 101.

In some embodiments, d_(x) is relatively large at image plane 102. Forexample, d_(x) at image plane 102 can be more than 1 mm (e.g., about 3mm or more, about 4 mm or more, about 5 mm or more, about 6 mm or more,about 7 mm or more, about 8 mm or more, about 9 mm or more, about 10 mmor more, about 11 mm or more, about 12 mm or more, about 13 mm or more,about 14 mm or more, about 15 mm or more, about 18 mm or more, about 20mm or more, about 25 mm or more). d_(x) can be about 100 mm or less(e.g., about 50 mm or less, about 30 mm or less). d_(y) at image plane102 can be in a range from about 0.5 mm to about 5 mm (e.g., about 1 mm,about 2 mm, about 3 mm, about 4 mm).

Typically, d_(r) at image plane 102 is about 10 mm or more. d_(r) canbe, for example, about 15 mm or more (e.g., about 20 mm or more, about25 mm or more, about 30 mm or more) at image plane 102. In someembodiments, d_(r) can be extremely large (e.g., about 1 m or more,about 5 m or more, about 10 m or more). In certain embodiments, thefield is rectangular in shape and d_(r) is infinite.

More generally, for other field shapes, projection objective 101 canhave a maximum field dimension of more than 1 mm (e.g., about 3 mm ormore, about 4 mm or more, about 5 mm or more, about 6 mm or more, about7 mm or more, about 8 mm or more, about 9 mm or more, about 10 mm ormore, about 11 mm or more, about 12 mm or more, about 13 mm or more,about 14 mm or more, about 15 mm or more, about 18 mm or more, about 20mm or more, about 25 mm or more) at image plane 102. In certainembodiments, projection objective has a maximum field dimension of nomore than about 100 mm (e.g., about 50 mm or less, about 30 mm or less).

In some embodiments, the image field shape can correspond (e.g., in oneor more dimensions) to the shape of die sites on a wafer that is exposedusing projection objective 101. For example, the image field can beshaped to reduce overscan when exposing the wafer. Overscan refers tothe need to scan the image field beyond the edge of a die site to exposethe entire site. Generally, this occurs where the shape of the imagefield does not conform to the shape of die site.

Overscan can be characterized by the ratio (e.g., expressed as apercentage) of the maximum distance between the leading edge of theimage field and the trailing edge of the die site at the position wherethe corners at the trailing edge of the die site are exposed. Referringto FIG. 7B, overscan corresponds to the ratio of d_(os) to d_(y), whered_(os) is the distance between the leading edge of image field 700 andthe trailing edge of die sites 720 at the position where corners 721 and722 are exposed. In certain embodiments, projection objective can haverelatively low overscan. For example, projection objective can have anoverscan of about 5% or less (e.g., about 4% or less, about 3% or less,about 2% or less, about 1% or less, about 0.5% or less, 0.1% or less).

In certain embodiments, projection objective 101 can be used with zerooverscan. For example, referring to FIG. 7C, in embodiments where animage field 730 is used to expose square die sites 740, scanning can beachieved with zero overscan.

Referring to FIG. 8, in general, projection objective 101 introduces anobject-image shift, d_(ois), that varies depending on the specificdesign of the projection objective. The object-image shift refers to thedistance of a point in the image field from the corresponding point inthe object field, as measured in the x-y plane. For projectionobjectives that have an optical axis (a common axis of rotationalsymmetry for each mirror in the projection objective) the object-imageshift can be calculated using the formula:

d _(ois) =h _(o)(1−M)

where h_(o) refers to the distance in the x-y plane of the central fieldpoint in the object field from the optical axis and M is the projectionobjective magnification ratio. For example, for a projection objectivehave a demagnification of 4× (i.e., M=0.25) and where the central fieldpoint is 120 mm from the optical axis, d_(ois) is 90 mm.

In some embodiments, projection objective 101 can have a relativelysmall object-image shift. For example, projection objective has zeroobject-image shift. Projection objectives having relatively small objectimage shifts can be have a relatively slim optical design. Furthermore,in embodiments that have zero object-image shift, projection objective101 can be rotated about the axis intersecting the central field pointsin the object and image fields without the central field pointtranslating with respect to, e.g., stage 130. This can be advantageouswhere, for example, metrology tools (e.g., detection optical systems,such as those disclosed in U.S. Pat. No. 6,240,158 B1) for inspectingand aligning wafers with respect to projection objective 101 are placedat a nominal position of the central field point because the centralfield point is not translated with respect to this position when theprojection objective rotates. Accordingly, zero object-image shift canfacilitate easier metrology and testing of projective objective 101where the projection objective is subject to rotations during the courseof operation.

In some embodiments, projection objective 101 has a d_(ois) of about 80mm or less (e.g., about 60 mm or less, about 50 mm or less, about 40 mmor less, about 30 mm or less, about 20 mm or less, about 15 mm or less,about 12 mm or less, about 10 mm or less, about 8 mm or less, about 5 mmor less, about 4 mm or less, about 3 mm or less, about 2 mm or less,about 1 mm or less).

Embodiments of projection objective 101 can have a relatively largeimage-side free working distance. The image-side free working distancerefers to the shortest distance between image plane 102 and the mirrorsurface of the mirror closest to image plane 102 that reflects imagedradiation. This is illustrated in FIG. 9, which shows a mirror 810 asthe closest mirror to image plane 102. Radiation reflects from surface811 of mirror 810. The image-side free working distance is denotedd_(w). In some embodiments, d_(w) is about 25 mm or more (e.g., about 30mm or more, about 35 mm or more, about 40 mm or more, about 45 mm ormore, about 50 mm or more about 55 mm or more, about 60 mm or more,about 65 mm or more). In certain embodiments, d_(w) is about 200 mm orless (e.g., about 150 mm or less, about 100 mm or less, about 50 mm orless). Projection objective 300, for example, has an image-side freeworking distance of approximately 45 mm. A relatively large workingdistance may be desirable because it can allow the surface of substrate150 to be positioned at image plane 102 without contacting the side ofmirror 810 facing image plane 102.

Analogously, the object-side free working distance refers to theshortest distance between object plane 103 and the mirror surface of thereflective side of the mirror in projection objective 101 closest toobject plane 103 that reflects imaged radiation. In some embodiments,projection objective 101 has a relatively large object-side free workingdistance. For example, projection objective 101 can have an object-sidefree working distance of about 50 mm or more (e.g., about 100 mm ormore, about 150 mm or more, about 200 mm or more, about 250 mm or more,about 300 mm or more, about 350 mm or more, about 400 mm or more, about450 mm or more, about 500 mm or more, about 550 mm or more, about 600 mmor more, about 650 mm or more, about 700 mm or more, about 750 mm ormore, about 800 mm or more, about 850 mm or more, about 900 mm or more,about 950 mm or more, about 1,000 mm or more). In certain embodiments,the object-side free working distance is no more than about 2,000 mm(e.g., about 1,500 mm or less, about 1,200 mm or less, about 1,000 mm orless). A relatively large object-side free working distance may beadvantageous in embodiments where access to the space between projectionobjective 101 and object plane 103 is desired. For example, inembodiments where reticle 140 is a reflective reticle, it is desirableto illuminate the reticle from the side that faces objective 101.Therefore, there should be sufficient space between projection objective101 and object plane 103 to allow the reticle to be illuminated byillumination system 120 at a desired illumination angle. Furthermore, ingeneral, a larger object-side free working distance allows flexibilityin design of the rest of tool, for example, by providing sufficientspace to mount other components (e.g. an uniformity filter) betweenprojection objective 101 and the support structure for reticle 140.

In general, projection objective 101 can be designed so that chief rayseither converge, diverge, or are substantially parallel to each other atreticle 140. Correspondingly, the location of the entrance pupil ofprojection objective 101 with respect to object plane 103 can vary. Forexample, where chief rays converge at reticle 140, the entrance pupil islocated on the image plane side of object plane 103. Conversely, wherethe chief rays diverge at reticle 140, object plane 103 is locatedbetween the entrance pupil and image plane 102. Furthermore, thedistance between object plane 103 and the entrance pupil can vary. Insome embodiments, the entrance pupil is located about 1 m or more (e.g.,about 2 m or more, about 3 m or more, about 4 m or more, about 5 m ormore, about 8 m or more, about 10 m or more) from object plane 103(measured along an axis perpendicular to object plane 103). In someembodiments, the entrance pupil is located at infinity with respect toobject plane 103. This corresponds to where the chief rays are parallelto each other at reticle 140.

In general, projection objective 101 can be designed using commerciallyavailable optical design software like ZEMAX, OSLO, or Code V.Typically, a design is started by specifying an initial projectionobjective design (e.g., arrangement of mirrors) along with parameterssuch as the radiation wavelength, field size and numerical aperture, forexample. The code then optimizes the design for specified opticalperformance criteria, such as, for example, wavefront error, distortion,telecentricity, and field curvature.

In certain embodiments, the initial projection objective is designatedby rotationally symmetric mirrors (e.g., spherical or asphericalmirrors) that are centered on an optical axis. Each mirror is thendecentered from the optical axis to a position where the mirrorsatisfies some pre-established criterion. For example, each mirror canbe decentered from the optical axis by and amount which minimizes thechief ray angle of incidence across the mirror for particular field. Inembodiments, mirrors can be decentered by about 5 mm or more (e.g.,about 10 mm or more, about 20 mm or more, about 30 mm or more, about 50mm or more). In certain embodiments, mirrors are decentered by about 200mm or less (e.g., about 180 mm or less, about 150 mm or less, about 120mm or less, about 100 mm or less).

Alternatively, or additionally, each mirror can be tilted to a positionwhere the mirror satisfies some pre-established criterion. The tiltrefers to the orientation of each mirrors symmetry axis with respect tothe optical axis of the initial configuration of the projectionobjective. Mirrors can be titled by about 1° or more (e.g., about 2° ormore, about 3° or more, about 4° or more, about 5° or more). In someembodiments, mirrors are tilted by about 20° or less (e.g., about 15° orless, about 12° or less, about 10° or less).

After decentering and/or tilting, a freeform shape for each mirror canbe determined to optimize the projection objective design for specifiedoptical performance criteria.

In addition to mirrors, projection objective 101 can include one or moreother components, such as one or more aperture stops. In general, theshape of the aperture stop can vary. Examples of aperture stops includecircular aperture stops, elliptical aperture stops, and/or polygonalaperture stops. Apertures stops can also be positioned so that the imageradiation makes a double pass or a single pass through the aperturestop. Aperture stops can be interchanged in projection objective 101and/or may have an adjustable aperture.

In some embodiments, projection objective 101 includes a field stop. Forexample, in embodiments where projective objective includes anintermediate image, the field stop can be positioned at or near theintermediate image.

Embodiments can include baffles (e.g., to shield the wafer from strayradiation). In some embodiments, projection objective 101 can includecomponents (e.g., interferometers) for monitoring changes in theposition of mirrors within the projection objective. This informationcan be used to adjust the mirrors to correct for any relative movementbetween the mirrors. Mirror adjustment can be automated. Examples ofsystems for monitoring/adjusting mirror position are disclosed in U.S.Pat. No. 6,549,270 B1.

Referring to FIG. 11, an embodiment of a projection objective 1200includes six mirrors 1210, 1220, 1230, 1240, 1250, and 1260, and has animage-side numerical aperture of 0.35 and an operating wavelength of13.5 nm. Mirrors 1210, 1220, 1230, 1240, 1250, and 1260 are all freeformmirrors. Projection objective 1200 images radiation from object plane103 to image plane 102 with a demagnification ratio of 4×. A referenceaxis 1205, orthogonal to object plane 103 and image plane 102 intersectscorresponding field points in the object and image fields. Thetracklength of projection objective 1200 is 1385 mm and the optical pathlength of imaged radiation is 4162 mm. Accordingly, the ratio of theoptical path length to tracklength is approximately 3.01. Projectionobjective 1200 has an aperture stop positioned at mirror 1220.

The entrance pupil of projection objective 1200 is at infinity withobject plane positioned between the entrance pupil and the mirrors. Thechief ray angle of the central field point at object plane 103 is 7°.The maximum variation of chief ray angles at object plane 103 is lessthan 0.06°. Projection objective 1200 is telecentric on the object side.

Projection objective 1200 has a rectangular field. The image-side fieldwidth, d_(x), is 26 mm. The image-side field length, d_(y), is 2 mm.Projection objective 1200 has zero object-image shift.

Projection objective 1200 provides an intermediate image between mirrors1240 and 1250.

The optical power of the mirrors in the order of the radiation path fromobject plane 103 to image plane 102 is as follows: mirror 1210 haspositive optical power; mirror 1220 has negative optical power; mirror1230 has positive optical power; mirror 1240 has positive optical power;mirror 1250 has negative optical power; and mirror 1260 has positiveoptical power.

The dimension of the footprint of each mirror, given as M_(x)×M_(y), isas follows: 250 mm×153 mm for mirror 1210; 70 mm×69 mm for mirror 1020;328 mm×153 mm for mirror 1230; 325 mm×112 mm for mirror 1240; 87 mm×75mm for mirror 1250; and 269 mm×238 mm for mirror 1260.

The chief ray angle of incidence for the central field point is 6.13°,10.61°, 8.65°, 8.26°, 14.22°, and 5.23° for mirrors 1210, 1220, 1230,1240, 1250, and 1260, respectively. The maximum angle of incidence,θ_(max) on each mirror for the meridional section is 6.53°, 11.63°,8.91°, 11.39°, 24.26°, and 7.44° for mirrors 1210, 1220, 1230, 1240,1250, and 1260, respectively. Δθ for mirrors 1210, 1220, 1230, 1240,1250, and 1260 are 1.07°, 3.64°, 1.74°, 7.44°, 21.70°, and 4.51°,respectively.

Mirrors 1210, 1220, 1250, and 1260 have freeboards that are more than 5mm and less than 25 mm. Mirror 1240 has positive chief ray anglemagnification while mirrors 1210, 1220, 1230, and 1250 have negativechief ray angle magnification.

The image-side free working distance of projection objective 1200 is 40mm. The object-side free working distance is 439 mm.

In projection objective 1200, d_(op−1)/d_(op−2) is 1.91. Furthermore,adjacent mirror pair 1240 and 1250 are separated by more than 50% of theprojection objective's tracklength. Further, the distance between mirror1210 and object plane 103 is more than 50% of the projection objective'stracklength.

Data for projection objective 1200 is presented in Table 2A and Table 2Bbelow. Table 2A presents optical data, while Table 2B presentsaspherical constants for each of the mirror surfaces. For the purposesof Table 2A and Table 2B, the mirror designations correlate as follows:mirror 1 (M1) corresponds to mirror 1210; mirror 2 (M2) corresponds tomirror 1220; mirror 3 (M3) corresponds to mirror 1230; mirror 4 (M4)corresponds to mirror 1240; mirror 5 (M5) corresponds to mirror 1250;and mirror 6 (M6) corresponds to mirror 1260.

TABLE 2A Surface Radius (mm) Thickness (mm) Mode Object INFINITY 836.375Mirror 1 −614.878 −397.397 REFL Mirror 2 −383.358 0.000 REFL STOPINFINITY 655.992 Mirror 3 −1204.989 −659.631 REFL Mirror 4 1885.915909.840 REFL Mirror 5 302.954 −308.805 REFL Mirror 6 403.492 348.850REFL Image INFINITY 0.000

TABLE 2B Coefficient M1 M2 M3 M4 M5 M6 K −6.673329E−01   −2.825442E−01  −1.843864E+00   2.076932E+00 3.340547E+00 1.990979E−01 Y −5.045837E−02  2.263660E−01 −1.277806E−01   −3.310548E−02   −1.935522E−01  1.783092E−02 X² 1.827144E−04 1.686990E−04 9.963384E−05 5.203052E−05−3.849892E−04   3.792405E−05 Y² 1.737812E−04 2.093994E−04−1.747764E−05   −7.184095E−05   −3.329705E−04   1.662759E−05 X²Y4.765150E−08 −1.595967E−06   −5.515151E−08   −8.752119E−10  1.213426E−06 5.552151E−08 Y³ 5.091508E−08 −1.231538E−06  −1.294839E−07   −1.939381E−07   1.502735E−06 9.165146E−08 X⁴−4.718889E−11   −6.941238E−09   −7.002011E−11   −5.996832E−11  −2.342602E−09   9.552648E−12 X²Y² −4.340357E−11   −7.827867E−09  −1.801185E−10   −7.139217E−11   −1.234047E−08   −1.611525E−10   Y⁴1.234053E−10 −3.130174E−09   −7.281275E−11   −1.598859E−10  −1.206604E−08   −1.662004E−10   X⁴Y 1.205203E−13 −6.495768E−11  −3.614883E−14   −4.344276E−14   2.268270E−11 2.930397E−13 X²Y³2.259661E−13 −4.304439E−11   −1.048629E−13   −7.811421E−16  2.977954E−11 8.493936E−13 Y⁵ −5.198478E−13   −1.485266E−11  5.022687E−15 −1.422459E−14   −1.556209E−11   4.051187E−13 X⁶−1.306395E−16   −4.159695E−14   0.000000E+00 −3.767576E−17  1.374773E−14 −9.890588E−17   X⁴Y² 8.838986E−17 1.462867E−14 0.000000E+00−1.369883E−16   −3.320990E−13   −1.312584E−15   X²Y⁴ −1.745854E−16  4.353978E−13 0.000000E+00 −7.920443E−17   −1.008910E−13  −2.069868E−15   Y⁶ 1.020155E−15 −1.927189E−13   0.000000E+00−3.431888E−17   −9.148646E−14   −6.650861E−16   X⁶Y 1.090627E−190.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 1.607288E−18 X⁴Y³−4.158749E−19   0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+004.652411E−18 X²Y⁵ −1.758731E−18   0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 4.087290E−18 Y⁷ −3.081679E−18   0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 9.802736E−19 X⁸ 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁶Y² 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁴Y⁴0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X²Y⁶ 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 Y⁸ 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X⁸Y 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁶Y³ 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁴Y50.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X²Y⁷ 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 Y⁹ 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 X¹⁰ 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁸Y² 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 X⁶Y⁴0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 X⁴Y⁶ 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 X²Y⁸ 0.000000E+00 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 Y¹⁰ 0.000000E+00 0.000000E+000.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 Nradius   1.00E+00  1.00E+00   1.00E+00   1.00E+00   1.00E+00   1.00E+00 Y-decenter−118.847 −100.000 100.000 24.472 −11.760 −37.772 X-rotation −7.782 7.3881.406 −2.140 −8.177 6.989

Referring to FIG. 12, an embodiment of a projection objective 1300includes six mirrors 1310, 1320, 1330, 1340, 1350, and 1360, and has animage-side numerical aperture of 0.35 and an operating wavelength of13.5 nm. Mirrors 1310, 1320, 1330, 1340, 1350, and 1360 are all freeformmirrors. Projection objective 1300 images radiation from object plane103 to image plane 102 with a demagnification ratio of 4×. Thetracklength of projection objective 1300 is 1500 mm and the optical pathlength of imaged radiation is 4093 mm. Accordingly, the ratio of theoptical path length to tracklength is approximately 2.73. Projectionobjective 1300 has an aperture stop positioned at mirror 1320.

The entrance pupil of projection objective 1300 is at infinity. Thechief ray angle of the central field point at object plane 103 is 7°.The maximum variation of chief ray angles at object plane 103 is lessthan 0.1°. Projection objective 1300 is telecentric on the object side.

Projection objective 1300 has a rectangular field. The image-side fieldwidth, d_(x), is 26 mm. The image-side field length, d_(y), is 2 mm.Projection objective 1000 has an object-image shift of 119 mm.

Projection objective 1300 provides an intermediate image between mirrors1340 and 1350.

The optical power of the mirrors in the order of the radiation path fromobject plane 103 to image plane 102 is as follows: mirror 1310 haspositive optical power; mirror 1320 has negative optical power; mirror1330 has positive optical power; mirror 1340 has positive optical power;mirror 1350 has negative optical power; and mirror 1360 has positiveoptical power.

The dimension of the footprint of each mirror, given as M_(x)×M_(y), isas follows: 271 mm×173 mm for mirror 1310; 69 mm×65 mm for mirror 1320;290 mm×115 mm for mirror 1330; 272 mm×66 mm for mirror 1340; 81 mm×67 mmfor mirror 1350; and 274 mm×243 mm for mirror 1360.

The chief ray angle of incidence for the central field point is 9.66°,12.15°, 9.10°, 5.45°, 13.31°, and 4.60° for mirrors 1310, 1320, 1330,1340, 1350, and 1360, respectively. The maximum angle of incidence,θ_(max), on each mirror for the meridional section is 11.20°, 15.46°,9.63°, 8.64°, 23.31°, and 6.17° for mirrors 1310, 1320, 1330, 1340,1350, and 1360, respectively. Δθ for mirrors 1310, 1320, 1330, 1340,1350, and 1360 are 3.25°, 7.32°, 1.57°, 6.92°, 21.18°, and 3.63°,respectively.

Mirror 1340 has positive chief ray angle magnification while mirrors1310, 1320, 1330, and 1350 have negative chief ray angle magnification.

The image-side free working distance of projection objective 1300 is 40mm. The object-side free working distance is 582 mm.

In projection objective 1300, d_(op−1)/d_(op−2) is 1.63. Furthermore,adjacent mirror pairs 1340 and 1350 is separated by more than 50% of theprojection objective's tracklength. Further, the distance between mirror1310 and object plane 103 is more than 50% of the projection objective'stracklength.

Data for projection objective 1300 is presented in Table 3A and Table 3Bbelow. Table 3A presents optical data, while Table 3B presentsaspherical constants for each of the mirror surfaces. For the purposesof Table 3A and Table 3B, the mirror designations correlate as follows:mirror 1 (M1) corresponds to mirror 1310; mirror 2 (M2) corresponds tomirror 1320; mirror 3 (M3) corresponds to mirror 1330; mirror 4 (M4)corresponds to mirror 1340; mirror 5 (M5) corresponds to mirror 1350;and mirror 6 (M6) corresponds to mirror 1360.

TABLE 3A Surface Radius (mm) Thickness (mm) Mode Object INFINITY 946.404Mirror 1 −605.890 −364.901 REFL Mirror 2 −368.417 0.000 REFL STOPINFINITY 626.468 Mirror 3 −1202.217 −556.441 REFL Mirror 4 1949.768808.432 REFL Mirror 5 276.499 −313.562 REFL Mirror 6 401.291 353.600REFL Image INFINITY 0.000

TABLE 3B Coefficient M1 M2 M3 M4 M5 M6 K −5.95606E−01   −1.82166E+00  −5.82444E−01   −2.38948E+00   3.35329E+00 1.67263E−01 Y 1.96214E−021.05243E−01 −1.91165E−01   −6.23536E−02   −4.99892E−02   1.30034E−02 X²1.71425E−04 1.61788E−04 8.52106E−05 7.49004E−05 −2.48914E−04  3.88103E−05 Y² 1.59322E−04 1.15506E−04 −1.78602E−05   −9.20778E−05  −2.00659E−04   4.01025E−05 X²Y 3.03035E−08 −8.08249E−07   −6.98999E−08  −6.74632E−08   7.56105E−07 5.29501E−09 Y³ 2.86899E−08 −3.26183E−07  −9.54345E−08   −1.51650E−07   2.54367E−07 8.86827E−09 X⁴ 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X²Y²0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00Y⁴ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 X⁴Y 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 X²Y³ 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 Y⁵ 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X⁶ 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X⁴Y²0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00X²Y⁴ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 Y⁶ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 X⁶Y 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 X⁴Y³ 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X²Y⁵ 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 Y⁷0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00X⁸ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 X⁶Y² 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 X⁴Y⁴ 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 X²Y⁶ 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 Y⁸ 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X⁸Y0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00X⁶Y³ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 X⁴Y5 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 X²Y⁷ 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 Y⁹ 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X¹⁰ 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 X⁸Y²0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00X⁶Y⁴ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 X⁴Y⁶ 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 X²Y⁸ 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 Y¹⁰ 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 Nradius   1.00E+00  1.00E+00   1.00E+00   1.00E+00   1.00E+00   1.00E+00 Y-decenter−200.000 −82.208 200.000 44.996 −23.759 −73.032 X-rotation −11.492 6.1534.904 −0.617 −3.814 7.081

Referring to FIG. 13, projection objective 1300 can be used in anoptical system 1401 that includes a light source 1405 and illuminationoptics including a collector unit 1415, a spectral purity filter 1425, afield facet mirror 1435, a pupil facet mirror 1445 and a focusing mirror1455. Light source 1405 is an EUV light source configured to provideradiation at 13.5 nm to the projection objective. Collector unit 1415gathers radiation from source 1405 and directs the radiation towardsspectral purity filter 1415 which filters incident radiation atwavelengths other than 13.5 nm and directs the radiation at 13.5 nmtowards field facet mirror 1435. Together with pupil facet mirror 1445and focusing mirror 1455, field facet mirror illuminates a reflectivereticle positioned at object plane 103 with radiation at 13.5 nm.

Other embodiments are in the claims.

1. An optical system, comprising: a plurality of optical elementsarranged to image radiation from an object field in an object plane toan image field in an image plane, wherein the system has an entrancepupil located more than 2.8 m from the object plane, a path of theradiation of the system has chief rays that are at an angle of 3° ormore with respect to a normal at the object plane, and the system is amicrolithography projection optical system.
 2. The optical system ofclaim 1, wherein the system is telecentric at the object plane.
 3. Theoptical system of claim 1, wherein the chief rays are at an angle of 4°or more with respect to the normal at the object plane.
 4. The opticalsystem of claim 1, further comprising a reflective object to be imagedin the object field.
 5. The optical system of claim 1, wherein thesystem is a catoptric projection objective.
 6. The optical system ofclaim 1, wherein at least one of the plurality of reflective elements isa reflective element having a rotationally asymmetric surface positionedin a path of the radiation, and the rotationally asymmetric surfacedeviates from a best-fit rotationally symmetric surface by λ or more atone or more locations, where λ is a wavelength of the radiation.
 7. Theoptical system of claim 6, wherein the best-fit rotationally asymmetricsurface deviates by about 0.1λ or less from a surface corresponding tothe equation:$z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {( {1 + k} )c^{2}r^{2}}}} + {\sum\limits_{j = 2}^{\alpha}{C_{j}x^{m}y^{n}}}}$where ${j = {\frac{( {m + n} )^{2} + m + {3n}}{2} + 1}},$ zis the sag of the surface parallel to an axis, c is the vertex curvatureand k is the conical constant, C_(j) is the coefficient of the monomialx^(m)y^(n), and α is an integer.
 8. The optical system of claim 6,wherein the rotationally asymmetric surface deviates from the best-fitrotationally symmetric surface by about 10λ or more at the one or morelocations.
 9. The optical system of claim 6, wherein the rotationallyasymmetric surface deviates from the best-fit rotationally symmetricsurface by about 20 nm or more at the one or more locations.
 10. Theoptical system of claim 1, wherein the plurality of reflective elementsdefine a meridional plane, and the plurality of reflective elements aremirror symmetric with respect to the meridional plane.
 11. The opticalsystem of claim 1, wherein the plurality of elements comprises twoelements that are reflective elements that have rotationally asymmetricsurfaces positioned in a path of the radiation.
 12. The optical systemof claim 1, wherein the plurality of reflective elements includes nomore than two reflective elements that have a positive chief ray anglemagnification.
 13. The optical system of claim 1, wherein the pluralityof reflective elements includes no more than one reflective element thathas a positive chief ray angle magnification.
 14. The optical system ofclaim 1, wherein the system has an image-side numerical aperture ofabout 0.2 or more.
 15. The system of claim 1, wherein the system has arectangular field at the image plane, and the rectangular field in eachof the orthogonal directions has a minimum dimension of about 1 mm ormore.
 16. The system of claim 1, wherein static distortion at the imagefield is about 10 nm or less.
 17. The system of claim 1, whereinwavefront error at the image field is about λ/14 or less.
 18. The systemof claim 1, wherein the chief rays are parallel to each other to within0.05° at the object plane.
 19. The system of claim 1, wherein theplurality of reflective elements is telecentric at the image plane. 20.The system of claim 1, wherein: for a meridional section of the opticalsystem, the chief ray of a central field point has a maximum angle ofincidence on a surface of each of the elements of θ degrees; the opticalsystem has an image side numeral aperture, NA, of more than 0.3; and aratio θ/NA is less than
 68. 21. The system of claim 1, wherein thesystem has an object-image shift of about 75 mm or less.
 22. The systemof claim 1, further comprising a radiation source configured to providethe radiation to an object plane.
 23. The system of claim 22, furthercomprising: an illumination system, comprising: one or more elementsarranged to direct radiation from the radiation source to an objectpositioned at the object plane; and an element positioned at a locationcorresponding to an entrance pupil of the optical system.
 24. A tool,comprising: an illumination system; and a projection optical system,comprising: a plurality of optical elements arranged to image radiationfrom an object field in an object plane of the projection optical systemto an image field in an image plane of the projection optical system,wherein the projection optical system has an entrance pupil located morethan 2.8 m from the object plane, a path of the radiation of the systemhas chief rays that are at an angle of 3° or more with respect to anormal at the object plane, and the tool is a microlithography tool. 25.The tool of claim 24, wherein the illumination system, comprises: one ormore elements arranged to direct radiation from the radiation source toan object positioned at the object plane; and an element positioned at alocation corresponding to an entrance pupil of the optical system
 26. Atool, comprising: an optical projection system comprising a plurality ofoptical elements arranged to image radiation having a wave length λ froman object field in an object plane of the optical projection system toan image field in an image plane of the optical projection system, theoptical projection system being telecentric at the object plane of theoptical projection system; a radiation source configured to provide theradiation at λ to the object plane of the optical projection system,where λ is 30 nm or less; and an illumination system having one or moreelements arranged to direct radiation from the radiation source to theobject plane of the optical projection system, wherein the tool is a EUVmicrolithography tool.
 27. A method, comprising using the tool of claim24 to product microstructured components.
 28. A method, comprising usingthe tool of claim 26 to product micro-structured components.